Membrane exchange humidifiers comprise a membrane that is permeable to water and/or water vapor. The fluid stream to be humidified (the dry stream) is directed over one side of the membrane while the fluid stream supplying the water (the wet stream) is directed over the opposing side of the membrane. (The terms “dry” and “wet” in this instance are relative terms; “dry” does not necessarily mean the complete absence of water, and “wet” does not necessarily mean saturation with water.) Water from the wet stream passes through the membrane thereby humidifying the dry stream. These humidifiers have been used for many purposes (for example, medical equipment, air conditioners).
Certain humidifier applications involve gaseous wet and dry streams whose compositions, except for the concentration of water, are similar. In such cases, membrane materials may be used that are significantly permeable not only to water but also to other components in the gaseous wet or dry streams. Additionally, certain humidifier applications involve wet streams that are simply liquid aqueous solutions or liquid water alone. In such cases, membrane materials may be used that are quite permeable to gases generally but not to liquid. Thus, in certain humidifier applications employing a liquid wet stream, microporous polymer membranes such as GORE-TEX® (polytetrafluoroethylene) may be employed.
However, if the humidifier application involves the use of wet and dry fluid streams of differing composition, then the membrane may preferably be selectively permeable to water. Otherwise, other components of the wet and dry fluid streams may mix undesirably via transport through the membrane. An example of a humidifier application in which the wet and dry fluid streams may be of differing composition is disclosed in U.S. patent application Ser. No. 09/108,156, filed Jun. 30, 1998, also owned by the assignee of the present application. In the '156 application, which is incorporated herein by reference in its entirety, a solid polymer fuel cell system is disclosed in which a reactant gas supply stream to the fuel cell may be adequately humidified using a reactant gas exhaust stream from the fuel cell via a membrane exchange humidifier apparatus. In particular embodiments, an air supply stream to the fuel cell may be adequately humidified using the wet oxygen-depleted air exhaust stream from the fuel cell. Typically, while the wet oxygen-depleted exhaust stream is predominantly gaseous, a portion consists of water in the liquid phase. In the Examples of the '156 application, NAFION® perfluorosulfonic acid membranes were used in the humidifiers. These membranes essentially prevent significant transmission of air or oxygen-depleted air therethrough.
In a solid polymer fuel cell, the ionic conductivity of the solid polymer electrolyte and the performance of the fuel cell are affected by the hydration level (both generally increasing with water content). As a result, fuel and/or oxidant reactant gas streams supplied to the fuel cell are typically humidified in order to maintain a sufficiently high level of hydration in the solid polymer electrolyte during operation.
The capacity of the reactant gases to absorb water vapor varies significantly with changes in temperature and pressure. If the reactant gas stream is humidified at a temperature higher than the fuel cell operating temperature, this can result in condensation of liquid water when the humidified reactant gas stream enters the fuel cell. Condensation may cause flooding in the electrodes, which may detrimentally affect fuel cell performance. Conversely, if the reactant gas stream is humidified at a temperature lower than the fuel cell operating temperature, the reduced water vapor content in the reactant gas stream could result in dehydration and damage to the solid polymer electrolyte. It is therefore preferred to humidify a reactant gas stream, typically at least the oxidant gas supply stream, at or close to the operating temperature and pressure within the fuel cell.
The solid polymer fuel cell system of the '156 application employs an effective arrangement for adequately humidifying and heating a reactant gas supply stream using a membrane exchange apparatus and a reactant gas exhaust stream from the fuel cell (typically at a slightly lower pressure than the supply stream). The reactant streams exiting the fuel cell (particularly the oxidant stream) typically contain sufficient water near the operating temperature of the fuel cell for purposes of humidification. This water in the reactant exhaust stream comes from water produced by the electrochemical reaction at the fuel cell cathode and from water vapor already present in the humidified stream delivered to the fuel cell. Use of an appropriate humidifier design and appropriate system operating parameters provides for adequate humidification of a reactant supply stream. For instance, certain values for the ratio (denoted by the dimensionless parameter R) of residence time divided by diffusion time for a hypothetical water molecule in a given chamber in the membrane exchange humidifier were found to be preferred. (By “hypothetical water molecule”, it is acknowledged that this ratio R is determined by a calculation based on apparatus characteristics and fluid flow rates and not by actual measurement of one or more water molecules.) To obtain the greatest flux of water through the membrane, the ratio R for the flows in the chambers may preferably be between about 0.75 and 3. This kind of humidifier is suitable for use with solid polymer fuel cell systems generally, including portable air-cooled systems that have no supply of liquid water coolant that can be used for humidification, as well as larger water-cooled systems.
A preferred configuration for a humidifier in one of the fuel cell systems described in the '156 application is a multiple plate-and-frame construction comprising a stack of plate-and-frame membrane exchange assemblies wherein each plate-and-frame membrane exchange assembly comprises a water permeable membrane sandwiched between two plates.
Although NAFION® and other similar materials are suitable as membrane materials, they also have certain disadvantages. For instance, NAFION® is not dimensionally stable under the varying humidity and temperature conditions of a fuel cell system (in which a humidifier may be exposed to humidity and temperature cycles ranging from ambient conditions during storage to conditions of full humidification at temperatures of about 100° C. or more). As a consequence, a NAFION® membrane may sag during operation and thus supporting ribs and/or bridges near the reactant stream inlet and outlet ports may be needed in a humidifier, thereby complicating design and construction. A requirement for bridges in particular can complicate construction. Further, if dimension changes from the dry state cannot readily be accommodated, it may be necessary to assemble such humidifiers with the membrane material in a wet state, a significant complication during assembly. Additionally, such materials are often not amenable to attaching via gluing or melt-bonding and thus compression type seals may need to be employed, again complicating design and assembly. Finally, such materials tend to be expensive. Thus, with regard to these disadvantages, other choices of membrane materials might be preferred.
Microporous polymer sheets comprising hydrophilic additives (for example, silica filled polyethylene sheets from companies such as PPG, Duramic, Entek, or Jungfer, silica filled latex sheet from Amerace, silica filled PVDF sheet from Elf Atochem, silica filled PVC sheet from Amersil) have been available commercially for some time and have found application as printing sheets and as battery separators. Such sheets may have good mechanical and water transmission properties but also may be significantly permeable to other fluids as well. Unlike many hydrophobic microporous sheet materials (for example, GORE-TEX®), these hydrophilic sheets may also be significantly permeable to liquid water and thus be considered unsuitable in certain applications (for example, wettable hydrophilic sheets that can transmit liquid water from the “wet” side to the “dry” side when the “dry” side is touched would be unsuitable as water proof breathable clothing).